FIGS. 13 and 14 show a heat sink of the type mentioned already known which comprises a main body 102 in the form of a vertical plate and having a heat receiving portion 103 for receiving the heat from a heat generating element and a heat radiating portion 104 integral with a lateral side of the heat receiving portion 103 and having an upper end projecting upward beyond the upper end of the heat receiving portion 103, and radiator fins 105 provided on one or each of opposite surfaces of the heat radiating portion 104. The heat sink main body 102 has a heat pipe portion 106 comprising a working fluid circuit 161 extending from the portion 103 to a lower region 104a of the heat radiating portion at the lateral side of the portion 103 and further to an upper region 104b of the heat radiating portion which region 104b is positioned at a higher level than the heat receiving portion 103, the circuit 161 having a working fluid (not shown) enclosed therein (see the publication of JP-A No. 8-186210). 
The working fluid circuit 161 of the heat sink 101 described has, for example, a pattern as shown in FIG. 14 and has substantially the same channel cross sectional area from portion to portion. The working fluid in a working fluid circuit portion 161B provided in the heat receiving portion 103 evaporates to a gas by receiving the heat emitted by the heat generating element. The gaseous working fluid flows through part of a working fluid circuit portion 161A provided in the heat radiating portion lower region 104a into a working fluid circuit portion 161C in the heat radiating portion upper region 104b, in which the fluid is subjected to heat exchange with air through the radiating fins 105, whereby the fluid is converted to a liquid on condensation. The liquid of working fluid flows through other part of the circuit portion 161A in the lower region 104a under gravity and returns to the circuit portion 161B in the heat receiving portion 103. In the case of the heat sink 101 described, however, the liquid of working fluid remains also in the circuit portion 161A in the lower region 104a because of structural reasons.
Even when the circuit portion 161A in the heat radiating portion lower region 104a permits the liquid of working fluid to remain therein locally, the liquid retaining part contributes nothing to heat radiation and is therefore a structurally useless part. Additionally in order to maintain the liquid of working fluid at a predetermined level, there is a need to increase the amount of working fluid enclosed in the circuit by an amount corresponding to the quantity of the liquid working fluid remaining in the circuit portion 161A.
Further since the radiator fins 105 are provided substantially over the entire area of one or each of opposite surfaces of the heat radiating portion 104, fins 105 are present also in the lower region 104a. Nevertheless, the fins 105 provided in the lower region 104a contribute almost nothing to heat radiation and are structurally useless. The presence of useless fins conversely increases the pressure loss of the air flowing between the fins 105, consequently leading to impaired heat radiation performance.
For use in cooling the heat generating elements, for example, of control devices of machine tools, an object of the present invention is to provide a heat sink of the heat pipe type which is free of problems such as the useless structural portion due to the stagnation of liquid working fluid that would occur in the working fluid circuit portion in the heat radiating portion lower region, an increase in the amount of working fluid enclosed in the circuit and an increased pressure loss of air and which exhibits high performance at a lower cost.